Hermetic seals between glasses and high expansion metals, like aluminum and stainless steel, are required for a wide variety of applications in the industrial and opto-electronic arts. Such seals will be employed in a great number of components including connectors and pyrotechnic devices. In the past, phosphate glasses, such as those disclosed in U.S. Pat. No. 4,202,700 (Wilder, Jr.) have been considered for such applications because they possess the required thermal expansion characteristics to make matching stress-free seals with various high expansion metals. Unfortunately, such glasses have generally limited usefulness because of the unacceptably poor aqueous durabilities which result in low weathering resistance and other problems which restrict the useful lifetimes of such seals.
It is known in the art to improve chemical durability and wear resistance of phosphate glasses by incorporating nitrogen into the glass structure. An example of such a procedure is disclosed in U.S. Pat. No. 4,455,384 (Day et al). Additionally, it has been shown that significant durability-enhancing nitrogen contents can be incorporated by melting the glass at temperatures of several hundred degrees above the glass transition temperature in an ammonia atmosphere (see, e.g., Marchand, J. Non-Cryst. Solids 56:173 (1983)). In one such treatment, it was shown that the glass aqueous durability could be increased by up to about 1,000 times its original level (see Rajaram et al, J. Am. Ceram. Soc. 70:203 (1987)).
However, it is also the case that the incorporation of nitrogen into the bulk glass structure also reduces the initial thermal expansion coefficient of the glass. This reduction in expansion coefficient is extremely detrimental to the ability of the glass to withstand the stresses of thermal expansion, which is crucial for the glass to be used in applications which require a glass-metal seal. As a result, phosphate glass treated by such high-temperature ammonia processes can no longer be used to provide stress-free hermetic seals on high expansion metals. It is thus highly desirable to develop a way of improving the chemical durability of phosphate glasses without significantly reducing the thermal expansion coefficient so that the improved phosphate glass can still be used in stress-free glass-metal hermetic seals.